This protocol describes a multicenter, expanded access, open-label, safety study of recombinant humanized anti-p185 HER2 monoclonal antibody (rhuMAb HER2) in approximately 500 patients with HER2 overexpression who have relapsed following two or more cytotoxic chemotherapy regimens for metastatic breast cancer. Due to the very limited drug supply available, eligible patients will be randomly selected by lottery based on an enrollment assumption of 100 patients per quarter for five quarters. Geographically diversed centers will be involved in the study in order to allow widespread availability of this new agent. Following screening for eligibility, the National Cancer Institute's Treatment Referral Center will refer patients to the closest participating center. The primary objectives of the study are a)to provide access to rhuMAb HER2 to patients with HER2 overexpressing metastatic breast cancer and b)to characterize the safety profile of rhuMAb HER2 alone or in combination with chemotherapy or hormonal treatment.